Pseudo skin flash on VMAT in breast radiotherapy: Optimization of virtual bolus thickness and HU values.

PURPOSE: Optimisation strategies for volumetric modulated arc therapy (VMAT) in most treatment planning systems for breast cancer do not account for patient positioning, breathing, or anatomical changes. To overcome this limitation, a pseudo-skin flash strategy using a virtual bolus has been proposed. Using this strategy, we determined optimal thickness and value of Hounsfield units (HU) assigned to the virtual bolus to ensure adequate CTV irradiation. MATERIALS AND METHODS: We modified the original computed tomography data (CT0) by adding combinations of thicknesses and densities of a virtual bolus on PTVs (CT') of seven bilateral breast cancer patients. Using a single optimization objective template, we obtained a VMAT plan on CT' and recalculated this on the CT0. Optimal CT' parameters were defined as those that minimized dose differences between CT' and CT0 plans regarding PTV and OAR dose-volume parameters. We studied bolus parameters regarding robustness by shifting the isocenter 5 and 10 mm in the breathing direction for each CT0 plan. RESULTS: The minimal dosimetric impact was between -400 and -600 HU depending on bolus thickness. OARs doses were not significantly affected. Best robustness was found for -500 HU and 15 mm bolus thickness against shifts of up to 10 mm in the breathing direction. CONCLUSION: Our results support a bolus thickness equal to the CTV-PTV margin plus 5 mm and a virtual bolus HU value around -500 and -400 depending on the bolus thickness chosen. These findings could play a useful role in maximisingrobustness and minimising the need for plan renormalization.

Needle migration and dosimetric impact in high-dose-rate brachytherapy for prostate cancer evaluated by repeated MRI.

PURPOSE: To quantify needle migration and dosimetric impact in high-dose-rate brachytherapy for prostate cancer and propose a threshold for needle migration. METHODS AND MATERIALS: Twenty-four high-risk prostate cancer patients treated with an HDR boost of 2 × 8.5 Gy were included. Patients received an MRI for planning (MRI1), before (MRI2), and after treatment (MRI3). Time from needle insertion to MRI3 was ∼3 hours. Needle migration was evaluated from coregistered images: MRI1-MRI2 and MRI1-MRI3. Dose volume histogram parameters from the treatment plan based on MRI1 were related to parameters based on needle positions in MRI2 or MRI3. Regression was used to model the average needle migration per implant and change in D90 clinical target volume, CTVprostate+3mm. The model fit was used for estimating the dosimetric impact in equivalent dose in 2 Gy fractions for dose levels of 6, 8.5, 10, 15, and 19 Gy. RESULTS: Needle migration was on average 2.2 ± 1.8 mm SD from MRI1-MRI2 and 5.0 ± 3.0 mm SD from MRI1-MRI3. D90 CTVprostate+3mm was robust toward average needle migration ≤3 mm, whereas for migration >3 mm D90 decreased by 4.5% per mm. A 3 mm of needle migration resulted in a decrease of 0.9, 1.7, 2.3, 4.8, and 7.6 equivalent dose in 2 Gy fractions for dose levels of 6, 8.5, 10, 15, and 19 Gy, respectively. CONCLUSIONS: Substantial needle migration in high-dose-rate brachytherapy occurs frequently in 1-3 hours following needle insertion. A 3-mm threshold of needle migration is proposed, but 2 mm may be considered for dose levels ≥15 Gy.